Field of the Invention
The present invention relates to an ultrasonic sensor including an ultrasonic transducer, and more particularly, to an ultrasonic sensor that detects the position of an object to be measured using a propagation phenomenon of ultrasonic waves.
Description of the Related Art
In the past, an ultrasonic sensor has been mounted on, for example, an automobile and has been used as a vehicle-periphery monitoring sensor that measures a distance between the automobile and an obstacle. Further, the ultrasonic sensor is applied to a preventive safety system that warns a driver when the distance between the automobile and an obstacle is close to a preset distance, automatically controls a brake when there is a collision hazard, and the like.
Here, the structure of the ultrasonic sensor will be described. The ultrasonic sensor has a structure where an ultrasonic transducer is assembled in a sensor case. The ultrasonic transducer includes a circular vibration surface, and includes a piezoelectric element. When a voltage is applied to the piezoelectric element, the piezoelectric element is vibrated by a piezoelectric effect and sends ultrasonic wave signals. When the piezoelectric element receives incoming ultrasonic wave signals, the piezoelectric element generates a voltage by an inverse piezoelectric effect. Furthermore, the ultrasonic sensor includes a circuit that is provided in a sensor case, drives the ultrasonic transducer, and processes a received signal voltage (for example, Patent Document 1).
[Patent Document 1] Japanese Patent No. 3628480
[Patent Document 2] JP-A-01-232629
However, for the efficient driving of the ultrasonic transducer of the ultrasonic sensor in the related art disclosed in Patent Document 1, high pressure cannot be applied to the ultrasonic transducer when the ultrasonic transducer is fixed. Accordingly, in the ultrasonic sensor in the related art, a gap is formed between the ultrasonic transducer and an elastic member holding the ultrasonic transducer and a gap is formed between the elastic member and the sensor case. For this reason, it is not possible to completely block the infiltration of water into the sensor case from the outside. Therefore, the influence of water, which infiltrates from the outside, easily exceeds the limit of a seal function of the ultrasonic sensor, and the infiltrating water reaches a position directly below the bottom of the ultrasonic transducer (the back side of the ultrasonic transducer). In addition, the influence of the water, which infiltrates from the outside and loses the destination thereof in the sensor case, generates a force that extrudes the ultrasonic transducer (or the elastic member and the like in addition to the ultrasonic transducer) to the outside of the sensor case.
Accordingly, when the mechanical strength of a line, such as a lead wire or a metal pin, or a connector for electrically connecting the ultrasonic transducer to a circuit board gradually reaches a limit, the line, such as a lead wire or a metal pin, or the connector is fractured. For this reason, the electrical connection between the ultrasonic transducer and the circuit board is damaged. As a result, the ultrasonic sensor cannot operate and breaks down. Even if the line, such as a lead wire or a metal pin, or the connector is not fractured, an excessive-length portion of the line embedded in a filler filling the sensor case is extruded and a water-infiltration passage is formed at a boundary between the filler and the line, such as a lead wire or a metal pin. For this reason, the infiltration of water to the circuit board included in the sensor case is allowed and causes a short circuit. As a result, the ultrasonic sensor breaks down.
Meanwhile, as a countermeasure against the above-mentioned breakdown, a structure in which a convex portion is provided on the outer surface of the ultrasonic transducer and is fitted to a holder in order to regulate the infiltration of water into the ultrasonic sensor or an ultrasonic switch having a structure that regulates the movement of the ultrasonic transducer is proposed (for example, see Patent Document 2).
However, since the shape of a general ultrasonic transducer is a substantially columnar shape, it is not possible to easily form a convex portion or a concave portion. For this reason, it is difficult to easily achieve the above-mentioned seal structure or the structure that regulates the movement of the ultrasonic transducer. Accordingly, when water, which is jetted with high pressure, directly hits the surface of the ultrasonic sensor, a risk that a breakdown is caused by the movement of the ultrasonic transducer is increased. In particular, when the ultrasonic sensor is mounted on an automobile, a scene in which an automobile is washed with high-pressure wash water is supposed. Accordingly, a risk that high-pressure wash water directly hits the ultrasonic sensor exposed to the surface of the automobile and a breakdown is caused by the movement of the ultrasonic transducer is high.